An electric motor has a base speed, up to which the maximum torque the motor can develop is substantially constant, whether the motor is running in the forward direction or reverse. Modern drive controllers are designed so that the motor can be run faster than the base speed, up to a maximum speed, albeit with reduced torque, with resulting better performance, or with the same performance from a more economical motor in terms of its rating and/or power consumption.
While the motor speed is lower than the base speed, the power supply can provide enough voltage in spite of the back electro-motive force (‘EMF’) that the motor generates. At motor speeds greater than the base speed however, field (magnetic flux) weakening is utilized to reduce the effective back EMF. In the case of a permanent magnet motor, for example, field weakening may be obtained by the field-producing stator current vector being displaced relative to its rotating axis.
The field weakening technique is especially important in the case of variable speed drives. Nowadays a vast majority of variable speed AC motors/drives is designed to be operated under conditions of voltage and current saturation, thus achieving a maximum utilization of current and voltage limits of all system components. This approach helps to achieve cost savings in an overall drive design.
Many field weakening techniques are known. U.S. Pat. No. 6,163,128 describes a method and drive system for controlling a permanent magnet synchronous machine. The controller described uses Q-axis current component error compared against a “pre-determined threshold” for operation in field-weakening as well as for transition from field-weakening back to normal operation and it is therefore necessary to determine that threshold. Also, if an absolute value of regulation error of the Q-axis current component controller is less than the “pre-determined threshold”, the field-weakening current component is discontinuously set to zero, giving rise to nonlinear behavior. In addition, the system described uses a filter, which can give rise to other performance problems.
U.S. Pat. No. 6,965,212 describes a method and apparatus for field weakening control in an AC motor drive system. The controller described uses the difference between voltage component limitation and the required Q-axis voltage component, notably its signum, for operation in field-weakening as well as for transition from field-weakening back to normal operation, which corresponds with a constant set-point. The field-weakening current component is generated using Q-axis current controller windup.